Molecular sieve zeolites have long been observed to demonstrate selective adsorption when in contact with a variety of adsorbable mixtures. This attribute may be utilized to effect a variety of separations, as for example, the removal of hydrocarbons from H.sub.2 -containing streams and the removal of nitrogen from air as well as other well known separations using pressure swing or vacuum swing processes. The adsorptive selectivity of the zeolite towards one or more components of a mixture must be maximized to optimize the efficiency of the desired separation. Assuming all other engineering factors remain constant, the adsorption characteristics of the material selected for the separation process influences both the production level and the purity of the gases produced. The gas industry is always looking for ways to improve bulk gas separation processes. Significant benefits are realized when the production rate of gas per volume of the adsorbent can be increased.
Workers in the field of air separation have concentrated the major portion of their efforts making process improvements of various kinds to the pressure swing adsorption (PSA) processes. Since the initial publication on PSA cycles in 1960 many improvements have been developed to improve the separation efficiency of the PSA processes. These are summarized in recent reviews by Keller and coworkers (Keller, G. E., Anderson, R. A. and Yon, C. M. "Adsorption" in Handbook of Separation Process Technology, ed. R. W. Rousseau, John Wiley and Sons, New York, p. 645 (1987), and Yang, R. T. "Gas Separation by Adsorption Processes", Butterworth's, London (1987)). It is clear from the vast amount of literature on adsorption that much more attention has been given to the process and relatively little to new adsorbents which may improve the PSA process. It is well known that the adsorbent's properties influence the efficiency of a PSA process and may even impact the choice of the specific process steps for a given separation. Most of the efforts to date have concentrated on A and X-type zeolites, largely due to their commercial availability.
U.S. Pat. No. 3,140,933 discloses the utility for air separation of the lithium form of all zeolites having an apparent pore size of at least 4 .ANG., and claims that LiX is most preferred due to its relatively high cation content.
Two Japanese publications describe the adsorption properties of Li mordenite (Minato, H.; Watanabe, M.; Scientific Paper General Education, Univ. of Tokyo, 1978, 28, 218; and Furuyama, S.; Katsumi, S.; J. Phys. Chem. 1982, 86, 2498-2503). These workers showed that compared to the sodium form, the lithium form of natural mordenite exhibits higher N.sub.2 capacity.
U.S. Pat. No. 4,544,378 discloses that highly exchanged CaX (Si/Al=1.23) with most of the calcium ions in the dehydrated/dehydroxylated state exhibit large N.sub.2 capacities and N.sub.2 /O.sub.2 selectivities. More recently it has been shown (Coe, C. G., Kuznicki, S. M., Srinivasan, R.; Jenkins, R. J. ACS Symposium Ser. 1988, 368 478-491) that low silica X zeolite (Si/Al=1.0) in the calcium form has more N.sub.2 -accessible calcium ions giving rise to high N.sub.2 capacities and N.sub.2 /O.sub.2 selectivities. However, these CaLSX adsorbents have poor hydrothermal stability and cannot be readily dehydrated on a commercial scale without undergoing a significant amount of cation hydrolysis leading to a loss of zeolite content and an inferior air separation adsorbent. Additionally, while both of the above calcium X-type adsorbents are useful for PSA processes that operate at subatmospheric conditions, they have limited working capacity in higher pressure applications.
U.S. Pat. No. 4,732,584 teaches that calcium chabazites have the highest N.sub.2 capacities at 1 atmosphere of any known adsorbent. However, the presence of calcium alters the overall shape of the isotherm and reduces the pressure where the adsorbent approaches saturation (i.e., where there is a very small change in capacity with relatively large incremental pressure changes above 0.5 atmospheres at 30.degree. C.). Therefore, the calcium form of chabazite probably does not have a large enough change in gas capacity between typical operating pressures (referred to as working capacity) for either VSA or PSA type applications to be practical.